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Functional Optical Zone of the Cornea

¹From the Laboratorio de Optica/Departmento Fisica, Universidad de Murcia, Murcia, Spain; ²LSU Eye Center, Louisiana State University Health Sciences Center, New Orleans, Louisiana; and ³Sarver and Associates, Inc., Carbondale, Illinois.

PURPOSE. When keratorefractive surgery is used to treat a central corneal diameter smaller than the resting pupil, visual symptoms of polyopia, ghosting, blur, haloes, and glare can be experienced. Progress has been made to enlarge the area of surgical treatment to extend beyond the photopic pupil; however, geometric limitations can pose restrictions to extend the treatment beyond the mesopic pupil diameter and can lead to impediments in night vision. The size of the treated area that has achieved good optical performance has been defined as the functional optical zone (FOZ). In this study the authors developed three objective methods to measure the FOZ.

METHODS. Corneal topography examination results from 1 eye of 34 unoperated normal eyes and 32 myopic eyes corrected by laser in situ keratomileusis (LASIK) were evaluated in three ways. First, a uniform axial power method (FOZ_A) assessed the area of the postoperative cornea that was within a ±0.5-D window centered on the mathematical mode. Second, FOZ was determined based on the corneal wavefront true RMS error as a function of the simulated pupil size (FOZ_R). Third, FOZ was determined from the radial MTF, established at the retinal plane as a function of pupil size (FOZ_M).

RESULTS. Means for each of the FOZ methods (FOZ_A, FOZ_R, and FOZ_M) were 7.6, 9.1, and 7.7 mm, respectively, for normal eyes. For LASIK-corrected eyes, these means were 6.0, 6.9, and 6.0 mm. Overall, an average decrease of 1.8 mm in the functional optical zone was found after the LASIK procedure. Correlations between the FOZ methods after LASIK showed acceptable and statistically significant values (R = 0.71, 0.70, and 0.61; P < 0.01).

CONCLUSIONS. These methods will be useful to more fully characterize corneal treatment profiles after keratorefractive surgery. Because of its ease of implementation, direct spatial correspondence to corneal topography, and good correlation to the other more computationally intensive methods, the semiempiric uniform axial power method (FOZ_A) appears to be most practical in use. The ability to measure the size of the FOZ should permit further evolution of keratorefractive surgical lasers and their algorithms to reduce the night vision impediments that can arise from functional optical zones that do not encompass the entire mesopic pupil.

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